In order to address the great issue of improving fuel efficiency in view of recently growing concern on global environment, an increasing number of automobiles have come to be attached with a continuously variable transmission (CVT), which are more efficient than a multi-stage automatic transmission (AT). The CVT often employs a metal push-belt, and is installed in a wide variety of vehicles of 0.6 to 3.5-litter displacement. A chain CVT, which is considered to exhibit higher efficiency, has come to be used in these days. Since the power is transmitted in these CVTs via a friction between a pulley and a belt or between a pulley and a chain, a large force is applied to force one of the components to the other in order to restrain a slippage between the components. CVTF (CVT Fluid) is used for lubrication between the belt and the pulley or between the chain and the pulley. Since reduction in a force for forcing one of these components to the other results in improvement in fuel efficiency, a high intermetal friction coefficient is required of the CVTF. In addition, in order to further improve the fuel efficiency, a mechanism for controlling a slippage in a lockup clutch installed with a torque converter is often used. Accordingly, most of the CVTF exhibits friction characteristics to a wet clutch. However, as the intermetal friction coefficient of the CVTF increases, vibrations and noises often occur between the pulley and the belt or between the pulley and the chain. In order to restrain the occurrence of the vibrations and noises, the CVTF is also demanded to exhibit excellent intermetal friction-coefficient/slipping-velocity characteristics (sometimes referred to as “intermetal μ-V characteristics” hereinafter) in addition to the high intermetal friction coefficient.
For instance, Patent Literature 1 discloses a combination of an alkaline earth metal salt with a high base number and an alkaline earth metal salt with a low base number, in which an imide compound and a phosphorus compound are blended, thereby enhancing the intermetal friction coefficient to improve the friction characteristics for a wet clutch. Patent Literature 2 discloses that an alkaline earth metal salt, a boron-containing succinimide, a triazole compound, an (alkyl)aryl phosphite, and an imide- or amine-friction modifier are blended to increase a transmission torque volume and to improve an abrasion resistance and friction characteristics to a wet clutch. Patent Literature 3 discloses that a phosphorus compound and succinimide are used to enhance the intermetal friction coefficient and restrain clogging of a wet clutch. Patent Literature 4 discloses that an organic acid metal salt, phosphorus compound, and succinimide are combined to enhance the intermetal friction coefficient and restrain clogging of a wet clutch. Patent Literature 5 discloses a compound for restraining noises. Specifically, a compound is provided by a combination of an alkaline earth metal sulfonate with a relatively low base number and a phosphate, in which a friction modifier including a sarcosine derivative and a reaction product of a carboxylic acid and amine are blended, thereby restraining a scratch noise.